The present invention relates to facsimile communication and more specifically to a variable speed facsimile transmitter which employs a storage mode photodetection array as a means of deriving a video line signal and method of operating the facsimile transmitter employing such photodetection arrays.
The present invention incorporates self-scanning photodetection arrays as a means of sensing visual information recorded on a printed document to be transmitted over long distances. The photodetection arrays utilize solid state photosensitive devices which operate in a storage mode. These devices, when operated in the storage mode with a junction reverse biased, have the characteristics of a capacitor. When the junction is open circuited the junction slowly discharges as electrons and holes are generated thermally and neutralize the stored charge on each side of the junction. With the application of light to the junction the discharge of the junction occurs much more rapidly and hence the junction may be used to sense light. Typically, the junction is recharged periodically and the recharging current is sensed; this current is a function of the total incident light on the junction, i.e. the time integral of the incident light. In many pattern recognition applications which utilize an array of photodiodes in a storage mode of operation, an elongated row of the photosensitive device is scanned electrically. Typically, the electrical circuit utilized to scan the photosensitive devices is incorporated on the same semiconductor substrate or chip as the photosensitive devices. The photodetection array incorporated in the present invention is commercially available from Reticon Corporation, U.S.A.
Because of the storage mode of operation with the magnitude of the available output varying as a function of the time integral of the quantity of incident light, and because of the possible difference between the scanning speed of the photosensitive device and the speed at which an electronic circuit processes the video signal into a format suitable for transmission, it is necessary to drive the photosensitive device such that a same line path of a printed document is scanned repeatedly until the processor is ready to process the output of the photosensitive device. Otherwise, the photosensitive device would reach the saturation level upon prolonged exposure to the sensed line. This requires that the photosensitive device be repeatedly discharged at intervals.
In practice, facsimile communication sytems are designed to operate on a number of transmission speeds to meet the user's requirements. Where the aforesaid photodetection array is employed as a means of scanning documents, the amplitude of the scanner output will vary depending on the transmission speed.
To keep the video output constant over the differing transmission speeds, the prior art system, disclosed in Japanese Patent Application No. 51-105221, laid open to public inspection on Sept. 17, 1976, discharges the photodetection array at a constant frequency which is a common multiple of the different scanning frequencies of the system. For example, assume the system have 3 Hz and 5 Hz scanning frequencies, the photodetector is discharged always at 15 Hz, regardless of the selected scanning frequency.
However, since the amplitude of the photodetector output decreases inversely with the discharge frequency, the video output of the prior art system is of small amplitude, which would require amplification of the signal or high performance optical system to reduce the loss of incident light.